Lloyds boss Horta-Osório's £1.48m bonus is linked to the government selling a third of its stake at a loss. Photograph: Leon Neal/AFP/Getty Images

The government could sell off its stakes in the bailed-out banks at prices substantially lower than expected after it linked the payment of a £1.5m bonus to António Horta-Osório, the boss of Lloyds Banking Group, to selling part of the taxpayers' stake.

Horta-Osório's bonus is linked to the government selling off a third of its 39% stake in the loss-making bank at share prices above 61p – considerably lower than the average price of 73.6p at which the taxpayer injected nearly £20bn into the bank in 2008.

The move reignited speculation that the government was considering a share sale in both Lloyds and Royal Bank of Scotland before the May 2015 general election, although the Treasury insisted there was no timetable for a selloff. RBS said last week that it could be ready to privatise part of the bank next year.

The 61p figure has never previously been published in an official document and the equivalent figure for Royal Bank of Scotland is 407p – below the 500p average price at which the taxpayer injected £45bn into the Edinburgh-based bank.

Ian Gordon, a banks analyst at the City broker Investec, described the 61p target as "a new 'contrived' break-even number for UK government accounting which conveniently ignores its average in-price of 73.6p".

Shares in Lloyds fell 2% on Friday to 53p after the bank reported a £570m loss for 2012 – better than the loss in 2011 of £3.5bn – after being hit by another £1.5bn provision for payment protection insurance (PPI) compensation. Its total bill for the mis-selling scandal is now £6.8bn. Lloyds is still paying out a total of £375m in bonuses despite the losses.

Sir Win Bischoff, the Lloyds chairman, said: "Given the trajectory we have, I would have thought it could be likely [that there could] be a share sale within the next two years". Lloyds stressed that Bischoff was not aware of a specific timetable.

The Unite union was angered by the bonus for Horta-Osório, who said he had wanted his payout to be linked to taxpayers getting their money back on their 39% stake in the bank. "My main objective is to get taxpayer money back," he said, adding he was "very confident" that this could be achieved.

The government introduced the requirement that the Lloyds boss could not get his bonus in the next five years unless it had sold at least 33% of its stake in the bank above 61p. The alternative stipulation, devised by Lloyds, is for a sale once a share price of 73.6p "has been reached for a given period of time". The shares would not be released to him until 2018.

Dominic Hook, national officer at Unite, said: "Lloyds is still making a loss and it's tainted by scandal. There is no justification for António Horta-Osório's share pot."

The bank said the Treasury had "informed us that 61p is the average price at which the equity support provided to Lloyds Banking Group is recorded in the public finances".

UK Financial Investments, which looks after the taxpayers' stakes in the bailed-out banks, has previously disclosed two prices for the Lloyds shares – the average price of 74p, which falls to 63p if the £2.5bn fee Lloyds paid to exit the UK asset protection scheme is included.

A Treasury spokesman said: "The government's strategy remains to see Lloyds continue the progress it has made in reforming itself into a strong and sustainable bank that supports the British economy, which in time can be returned to full private ownership ... [T]here is still work to be done as it continues to deal with the legacy of the past."

The 61p is based on the stock market price of the Lloyds shares when the government bought them rather than the actual price that the government paid, and has been in the government's accounts since 2010 when the £65bn ploughed into both banks was valued at £53.8bn.

The scale of the PPI provision means that Lloyds is considering clawing back bonuses from 13 or so former and current executives. The former Lloyds boss Eric Daniels, who has already had £580,000 of his 2010 bonus clawed back, is thought to be among them. Daniels had recently said he believed the bank "was on the side of the angels" in handling PPI. Horta-Osório, who took over from him two years ago when he was ]hired from Santander on an £8m-a-year pay package, said: "In that case, I can only imagine what would have happened if we were not on the side of the angels."

In return for the taxpayer bailout, the European commission has required Lloyds to sell off 632 branches. While negotiations are under way with the Co-op, Horta-Osório said the mutual was in discussions with the Financial Services Authority about its capital position. But it is also working on plans for a stock market flotation of the branches, which will take the centuries-old brand of Trustee Savings Bank and bring a rebranded Lloyds back on the high street.